I've experienced this on many of the different Bimmerpost forums that I frequent. I use either Chrome or Firefox and have the same issue. Not sure why, because I don't have this problem on other forums that are running vBulletin.

I think it's either a problem with the database (which wouldn't be too surprising) or an artifact of posts being deleted. My theory is that a thread keeps a separate ongoing counter of all the posts, which is used to determine the number of pages based on your posts per page setting. That way it's not querying all the posts in the thread to determine the number of pages, but rather just using a single integer value instead. If that counter gets out of sync, it will be wrong. My theory anyways.